Typically, finely divided dispersions of metal oxides and/or hydroxides are obtained by e.g., joint precipitation of hydroxides and calcination of the precipitates obtained. Such methods have been described in Tonindustrie Zeitung 94 (1970) No. 2, p. 45 ff., and Journal of Materials Science 16 (1981), pp. 3447-3451. Using these methods it is possible to obtain finely divided dispersions of the respective materials. However, such methods are very expensive, since it is necessary first to separately prepare solutions of salts of the respective metals, including aluminum. Additional problems arise from the fact that individual components do not precipitate at the same pH value and thus insufficient mixing of the components may result.
A further disadvantage of the known processes is the accumulation of a large quantity of salts in the neutralization process. The salts are required to effect the precipitation of hydroxides. These salts are retained in the precipitation and can be recovered only by meticulous washing (see U.S. Pat. No. 3,586,653).
Another method of preparing hydroxide dispersions is the "sol-gel" technique, used e.g. with the elements uranium, zirconium, thorium, magnesium, zinc, nickel, aluminum, and titanium (Am. Ceramic Soc. Bull. 51 (1972), pp. 158-161). The particle size in the sol is very small and the solids content usually very low. Dried product can be obtained only after removing a large quantity of water at a high cost. Moreover, the dried product has a very low bulk density.
Another method for preparation of mutual dispersions of different oxides is the hydrolysis of organometallic compounds, known for mixtures of zirconium, titanium, and aluminum oxides. This process is described in Journal Am. Ceramic Soc. 50 (1967), pp. 532-537.
One objective of the present invention is to prepare finely divided homogeneous dispersions of metal oxides and/or hydroxides having an aluminum hydroxide coating with a low water content, i.e. with high solids concentration in the filter cake, by means of a simple and environmentally acceptable process.